As a suspension spring for use in a car, there are used a leaf spring and a spring which is made of a round bar and to which torsion stress is to be applied (a torsion bar, a stabilizer, a coil spring, etc., hereinafter referred to as the spring made of round bar, appropriately). The coil spring is generally used in passenger cars, and the leaf spring is used in trucks. The leaf spring and the spring made of round bar are each one of the large parts in terms of weight among the chassis parts and those parts are continuously researched and developed for higher strength for weight saving conventionally.
To achieve higher strength, it is particularly important to improve fatigue strength, and hardening of the steel is one of the measures for that.
However, as to both of the spring made of round bar and the leaf spring, it is known that if tensile strength is increased by increasing hardness, fatigue strength will be effectively improved in an ordinary environment, while in a corrosive environment, if tensile strength is increased by increasing hardness, fatigue strength will be adversely significantly decreased.
Accordingly, the most significant problem in the conventional developments has been that the countermeasure for improving the tensile strength by simply improving the hardness will not lead to the solution of the problems. Further, although the leaf spring and the spring made of round bar are generally painted when used, there is a possibility that the surface painting of the springs is damaged during driving due to hit by stone, etc., since they are put on cars at a position near the ground, and corrosion may be gradually progressed from the damaged sections, and which may cause breakage in some cases. Still further, a snow melting agent contributing to corrosion is occasionally dispersed on the road in winter to prevent road surface freezing.
For those reasons, there have been strong requirement for development of steel which are hardly lowered in corrosion fatigue strength even if their hardness is improved.
Study has conventionally been conducted in many ways on a decrease in strength, especially, in a decrease in fatigue characteristics in the corrosive environment; in fact a lot of documents etc. have made clear that hydrogen generated as corrosion progresses enters steel and contributes to embrittlement of the steel. As the countermeasures, technologies disclosed in, for example, the following Patent Documents 1 to 3 are reported.